1. Field of the Invention
This invention relates to compactability measurement for sand in a sand casting foundry system. More specifically, this invention relates to non-contact continuous measurement of sand compactability used in a foundry sand casting system.
2. Description of the Prior Art
The use of sand casting for a foundry system is well known in the art. In particular, the sand is fed into a molding machine and serves as a mold for casting a molten metal article. Following the hardening of the molten metal article, most of the sand is recycled into a sand mixer or muller wherein water, binder, and additional sand are added. The additional sand replaces sand which has been lost in the recycling operation, as by sticking to the metal article produced by the molding process. The recycled sand is fed from the sand mixer into the molding machine to complete the loop.
One of the problems associated with sand casting is the need to insure that the sand is suitable for use in the molding process. If the sand does not have the proper characteristics for functioning in the molding process, all or part of a production line may have to be shut down in order to remedy the problem. Additionally, materials may be wasted in producing a metallic article which is not acceptable.
One of the more important characteristics of the sand used in a sand casting operation is the compactability of the sand. The compactability, which usually is between 35 and 55 percent for most foundry operations, is a measure of how much the sand can be compacted during the molding process. Compactability may be expressed as a ratio of the difference between the compacted sand density and the non-compacted sand density to the compacted sand density.
Compactability in the industry has traditionally been measured by taking a sample of sand either before or after preparation for molding. Generally, a prepared sample is taken somewhere between the mixer and the molding machine. The sample is screened or fluffed into a standard cylinder and raked level on the top. The sand is then rammed three times with a two kilogram weight. Percent compactability is computed by measuring the travel of the ram.
It should be understood that the compactability measurement by ramming is actually taking a ratio of noncompacted volume to compacted volume. However, for a given amount of material, the percentage change in volume will be equal to the percentage change in density. This is true because the density is defined as mass over volume and, therefore, for a given amount of mass, the density times the volume will always be constant. Accordingly, the ram technique allows one to measure compactability by measuring a change in volume.
The prior art further includes numerous techniques for measuring various characteristics of materials. The following patents disclose several such techniques:
______________________________________ U.S. Pat. No. Inventor (s) Date Issued ______________________________________ 3,534,260 Walker Oct. 13, 1970 3,136,010 Dietert et al June 9, 1964 3,460,030 Brunton et al Aug. 5, 1969 3,693,079 Walker Sept. 19, 1972 3,600,574 Glaza et al Aug. 17, 1971 3,223,964 Stadlin Dec. 14, 1965 2,890,347 McCormick June 9, 1959 2,679,317 Roop May 25, 1954 3,510,374 Walker May 5, 1970 ______________________________________
The Roop patent discloses an X-ray system which tracks property changes as a particular object or material degrades. A prior measurement of the same property of the object may be used as a reference for comparison purposes.
The McCormick patent shows the use of an X-ray measuring system wherein the absorption of X-rays in the test material is compared with the absorption in a standard specimen.
The Dietert et al patent shows a sand casting moldability measurement system using a balance plate. The measurement of the moldability, which is defined therein as dependent upon the amount of sand which passes through a screen relative to the amount of sand which passes completely over the screen, is used for controlling the addition of an additive to the sand mixer. As an alternative to moving the sand across a vibrating screen for determining moldability, the sand is compressed and the bending strength of the compressed sand is measured to provide an indirect measurement of the sand moldability.
The Stadlin patent discloses an ultrasonic measurement system for determining height of a granular material.
The Brunton et al patent discloses a moisture percentage measurement using both microwaves and gamma rays. This system computes a ratio of a voltage dependent upon the sand moisture content and a voltage dependent upon the moist weight of the sand.
The Walker '374 patent discloses a feedback system for controlling and gauging a particular property in a processed material. The property or characteristic of a material used in a rubber calendaring process is gauged and the measured value is used for adjusting the material property as a function of a difference between the measured property and a target or desired value of the property.
The Walker '260 and '079 patents disclose the use of gamma rays in combination with microwaves for determining the moisture content of different materials.
The Glaza et al. patent discloses the use of a gamma detector in a sand chute. Specifically, a gamma source is disposed in a probe which extends into a sand chute and gamma detectors are located immediately outside of the chute. The gamma rays are used to determine the sand density, whereas a neutron source and neutron detector is used to detect the amount of water in the sand.
In addition to checking the compactability by the ram measurement test discussed above, the prior art further includes taking a sample of sand off the conveyor belt between a sand mixer and the molding machine and making a drop weight. Alternately, the sample is run through a groove wheel with a second displaceable wheel which rides in the groove, the displacement of the second wheel being dependent upon the compactability of the sand.
Although the prior art techniques have been generally useful at determining particular properties or characteristics of materials, they have been generally subject to one or more serious disadvantages.
One disadvantage common to many prior art measurement techniques is the requirement for removing a sample of the sand from the molding production line. If using a compactability measurement technique which actually compacts the sand, the sand must of necessity be removed from passage from the sand mixer to the molding machine. As is well known in the field, the molding machine requires loose or uncompacted sand to be fed into it.
In addition to the disadvantage of separating out sand from passage between the sand mixer and the molding machine, the prior art techniques which depend upon the mechanical compaction of the sand also are generally batch type techniques. That is, they do not provide a continuous measurement of compactability. Instead, the ram type compaction measurement technique and similar mechanical compaction methods usually require a set amount of sand to be fed into a chamber which is then compacted, compaction measurements being output only at the discrete times and for the discrete sample amounts of the sands which are measured during that particular batch. Depending upon the frequency of compactability measurements and whether the sample is truly representative of the overall sand, the measurement results may be of questionable accuracy.
A further disadvantage common to ram or other mechanical compactability measurement systems is that they require mechanically movable parts (e.g., the compressing ram) which require significant amounts of energy for their operation and which are subject to mechanical breakdown.
Another common disadvantage to many prior art techniques is the need for a human operator to initiate a measurement operation.
Although the prior art techniques discussed above include numerous techniques which are continuous measurement systems and avoid some of the disadvantages heretofore discussed, these prior art techniques do not determine the compactability of the sand. The compactability of the sand is dependent upon moisture content and numerous other factors such as its chemical composition and grain size. However, because the compactability is a relatively complex function of the moisture content, one cannot readily determine the compactability from knowledge of the moisture content alone.